1. Field of the Invention
The present invention relates to concrete and specifically to ready-mixed concrete and other applications in which considerable time may elapse between the mixing of the concrete ingredients and the placement of the concrete mixture into a suitable form or mold. More particularly, the present invention concerns a method for preparing concrete containing a chemical admix wherein the chemical admix is slurried with water and a pozzolan material prior to its incorporation into the concrete mixture. The method results in a concrete which has prolonged workability.
2. Description of the Prior Art
The binding properties of cement are the results of a complex series of chemical reactions which occur primarily between its lime and silicate constituents in the presence of water. As soon as the cement and water are mixed, these chemical reactions begin. At any given time the properties of the paste, mortar, or concrete in which the cement is incorporated are significantly affected by the extent to which these chemical reactions have proceeded.
In many commercial situations the concrete ingredients, cement, fine aggregate, water, and coarse aggregate, are added to a truck at a central location and transported with or without mixing to the site where the concrete is to be used. Unfortunately, for many reasons the time between the initial mixing of the ingredients and the time when the ready-mixed concrete can be placed into the form at the point of use cannot be closely controlled. Since the cementitious reactions have already begun at the central mixing locations, if an unduly long delay occurs, the reaction may have proceeded to such an extent that the concrete lacks sufficient plasticity to be conveniently handled, and the concrete hardens too rapidly in the form to allow proper consolidation and finishing. If the concrete has become so unworkable that it cannot be utilized, the truck load of concrete will be rejected, and the economic loss therefore will be assessed to either the supplier or the customer depending on the cause.
To alleviate the problem of rapid hardening in the form, additional chemicals, commonly referred to as chemical admixes, have been added to the concrete mixture. Although these chemical admixes are effective in altering the hardening time of the concrete in the form, they do not reduce the slump loss of the concrete under normal conditions during transportation. Indeed, some chemical admixes have the deleterious side effect of increasing the loss in the workability of the concrete during transit.
In an attempt to correct the slump loss of the concrete during shipment and to increase its workability, additional water is frequently added to the concrete at the point of use. Although this procedure does make the concrete somewhat more manageable, the addition of excess water reduces the strength of the concrete. Moreover, since it is difficult to mix the additional water throughout the truck load, the resulting concrete lacks homogeneity, which may result in a structure having localized areas of weakness or low compressive strength.
Thus, the prior art has failed to provide a method of preparing concrete containing a chemical admix without incurring the disadvantages associated therewith.